This invention is concerned with a drive arrangement and more especially, but not exclusively, a drive arrangement for use in a folding machine of a type used in the shoe machinery and allied trades, i.e. a machine for folding the edge of a workpiece over upon itself. More particularly, the drive arrangement with which the present invention is concerned is of the type. comprising an a.c. induction motor having a rotary output shaft, and a drive control by which the speed of rotation of the shaft is controlled. and which comprises a signalling arrangement for supplying a signal having a value corresponding to the desired speed of rotation of the motor output shaft, the arrangement including a controller for supplying a signal having a value which can be varied according to the desired speed of rotation of the motor output shaft, and which receives a control signal corresponding to the signal from the controller and, in response to such signal being received, supplies a corresponding drive signal to the a.c. induction motor. One such drive arrangement is disclosed for use in a sewing machine in U.S. Pat. No. 4,627,370.
The advantage of when using an a.c. induction motor controlled through an inverter is essentially one of cheapness, in particular as compared with, say, a d.c. servomotor, without significant loss of accuracy of control. In many instances, furthermore, the degree of control is acceptable but in general (and indeed in the machine described in the aformentioned U.S. Pat. No. 4,627,370) the control of the speed of operation of the machine is determined by the signal from the controller without any facility for other inputs which may modify the operating speed.
In the machine of the aforementioned patent, the controller is operator actuatable, more particularly treadle controlled, in order to leave the hands of the operator free for manipulating the workpiece. Using such a system, the speed of operation of the machine can be varied over a range of speeds up to a maximum, which normally will correspond with the full depression of the treadle. Where an operator is highly experienced, such a speed control is acceptable. In the case of a less experienced operator, or indeed a learner, however, it is often desirable to restrict the maximum operating speed of the machine to a lower value and indeed such an arrangement has already been proposed in respond of a folding machine. In this machine proposal, however, operation of the threadle is effective to control the speed regardless of the setting of the maximum speed, such that once the set desired maximum has been achieved, further depression of the treadle is ineffective to increase the speed; that is to say, if for example the maximum speed is 50% of the overall maximum, then only 50% of the treadle movement will be effective to vary the speed, so that the greater reduction is in the desired maximum speed, the less the "resolution" on the treadle. This is particularly undesirable in the case of a learner or an inexperienced operator.